outputSchema fix - return CallToolResult on structuredContent not returned or structuredContent validation error

Author: KKonstantinov

package-lock.json

@@ -947,7 +947,7 @@ describe('tool()', () => {
             expect.arrayContaining([
                 {
                     type: 'text',
-                    text: expect.stringContaining('Invalid arguments')
+                    text: expect.stringContaining('Input validation error: Invalid arguments for tool test')
                 }
             ])
         );
@@ -971,7 +971,7 @@ describe('tool()', () => {
             expect.arrayContaining([
                 {
                     type: 'text',
-                    text: expect.stringContaining('Invalid arguments')
+                    text: expect.stringContaining('Input validation error: Invalid arguments for tool test (new api)')
                 }
             ])
         );
@@ -1168,14 +1168,23 @@ describe('tool()', () => {
         await Promise.all([client.connect(clientTransport), mcpServer.server.connect(serverTransport)]);
 
         // Call the tool and expect it to throw an error
-        await expect(
-            client.callTool({
+        const result = 
+            await client.callTool({
                 name: 'test',
                 arguments: {
                     input: 'hello'
                 }
-            })
-        ).rejects.toThrow(/Tool test has an output schema but no structured content was provided/);
+            });
+
+        expect(result.isError).toBe(true);
+        expect(result.content).toEqual(
+            expect.arrayContaining([
+                {
+                    type: 'text',
+                    text: expect.stringContaining('Output validation error: Tool test has an output schema but no structured content was provided')
+                }
+            ])
+        );
     });
     /***
      * Test: Tool with Output Schema Must Provide Structured Content
@@ -1290,14 +1299,22 @@ describe('tool()', () => {
         await Promise.all([client.connect(clientTransport), mcpServer.server.connect(serverTransport)]);
 
         // Call the tool and expect it to throw a server-side validation error
-        await expect(
-            client.callTool({
+        const result = await client.callTool({
                 name: 'test',
                 arguments: {
                     input: 'hello'
                 }
-            })
-        ).rejects.toThrow(/Invalid structured content for tool test/);
+            });
+
+        expect(result.isError).toBe(true);
+        expect(result.content).toEqual(
+            expect.arrayContaining([
+                {
+                    type: 'text',
+                    text: expect.stringContaining('Output validation error: Invalid structured content for tool test')
+                }
+            ])
+        );
     });
 
     /***

src/server/mcp.test.ts

@@ -144,7 +144,7 @@ export class McpServer {
                     if (!parseResult.success) {
                         throw new McpError(
                             ErrorCode.InvalidParams,
-                            `Invalid arguments for tool ${request.params.name}: ${parseResult.error.message}`
+                            `Input validation error: Invalid arguments for tool ${request.params.name}: ${parseResult.error.message}`
                         );
                     }
 
@@ -155,27 +155,29 @@ export class McpServer {
                     const cb = tool.callback as ToolCallback<undefined>;
                     result = await Promise.resolve(cb(extra));
                 }
+
+                if (tool.outputSchema && !result.isError) {
+                    if (!result.structuredContent) {
+                        throw new McpError(
+                            ErrorCode.InvalidParams,
+                            `Output validation error: Tool ${request.params.name} has an output schema but no structured content was provided`
+                        );
+                    }
+    
+                    // if the tool has an output schema, validate structured content
+                    const parseResult = await tool.outputSchema.safeParseAsync(result.structuredContent);
+                    if (!parseResult.success) {
+                        throw new McpError(
+                            ErrorCode.InvalidParams,
+                            `Output validation error: Invalid structured content for tool ${request.params.name}: ${parseResult.error.message}`
+                        );
+                    }
+                }
             } catch (error) {
                 return this.createToolError(error instanceof Error ? error.message : String(error));
             }
 
-            if (tool.outputSchema && !result.isError) {
-                if (!result.structuredContent) {
-                    throw new McpError(
-                        ErrorCode.InvalidParams,
-                        `Tool ${request.params.name} has an output schema but no structured content was provided`
-                    );
-                }
 
-                // if the tool has an output schema, validate structured content
-                const parseResult = await tool.outputSchema.safeParseAsync(result.structuredContent);
-                if (!parseResult.success) {
-                    throw new McpError(
-                        ErrorCode.InvalidParams,
-                        `Invalid structured content for tool ${request.params.name}: ${parseResult.error.message}`
-                    );
-                }
-            }
 
             return result;
         });